1. Field of the Invention
This invention relates to a measuring apparatus for applying light, for example, to an object to be examined and measuring scattered light and fluorescence radiated from the object to be examined.
2. Related Background Art
Flow cytometers, particle counters, foreign substance defect inspecting apparatuses, etc. are known as examples of a measuring apparatus for applying a radiation beam such as a laser beam to an object to be examined and measuring scattered light and fluorescence radiated from the object to be examined, and have been widely used in the fields of biology, medical treatment, semiconductors and other industries.
Generally, in a precise optical measuring apparatus, a laser beam excellent in monochromatism and light condensing property is used as a light source. Usually, however, the pointing of the laser beam minutely fluctuates at a frequency of 100 Hz or more and at the same time, this pointing also fluctuates gently due to the heat contraction of a laser holding mechanism. Consequently, in the optical system of the prior-art measuring apparatus, accurate control of the applied position of the laser beam has been difficult and further improvement in measurement accuracy has not been easy.
Also, if a fluid system for flowing a particle is unstable, the particle flowing through a flow path will fluctuate from its original position in a direction orthogonal to the flow path. In this case, the intensity of the laser beam assumes Gaussian distribution and therefore, when the position of the particle fluctuates relative to the applied laser beam, the intensity of the laser beam applied to the particle changes and thus, the intensity of scattered light radiated from the particle or the fluorescence excited by the laser beam becomes unstable and accurate measurement of the particle becomes difficult.
So, in order to solve this problem, the spot shape of the applied laser beam has heretofore been made into an elliptical shape having a major axis in a direction orthogonal to the flow of the particle. Thus, the intensity distribution of the applied laser beam spreads and the variation in the intensity of the applied beam in a direction orthogonal to the flow decreases and therefore, even if the position of the particle somewhat moves in said direction relative to the applied laser beam, the fluctuation of the intensity of the applied laser beam impinging on the particle can be suppressed.
In this method, however, the cross-sectional area of the applied laser beam becomes large and therefore, the density of the light energy applied to the particle decreases and as a result, the intensities of scattered light and fluorescence radiated from the particle become weak and the S/N ratio of a signal detected by a detector becomes low. To compensate for this, a laser source emitting a powerful laser beam can be used, but this will lead to the bulkiness and increased cost of the apparatus.